Re: [tree-ssa/mainline PATCH] fold "test"==0 into constant

[Toon Moene <toon at moene dot indiv dot nluug dot nl>]

Roger Sayle wrote:

> Longer-term, I think what constant folding really needs are two
> functions tree_expr_lower_bound and tree_expr_upper_bound, where we
> can implement some form of interval arithmetic.  Then the two current
> functions tree_expr_nonnegative_p and tree_expr_nonzero_p can easily
> be implemented by testing the returned bounds against zero.  This will
> provide for more accurate implementations, but also enable many more
> optimizations, for example, comparisons against any integer constant,
> or comparisons between expressions with non-overlapping bounds.
>
> For example, we could remove bounds checks from "switch (x & 7)"...

Just this week I ran into the following:

An important part of our Numerical Weather Forecasting Program has to 
know about the saturation (water) vapor pressure as function of temperature.

This relation is well known, but (because it involves the exponential 
function) quite expensive to compute - so we tabulate the results in the 
relevant temperature interval (173 - 373 Kelvin) in an array for every 
0.01 Kelvin.

Therefore, basically, in the code, you're left with:

      psat(t) = tpsat(nint(t*100.0))

Now it would be very nice if we could indicate to the backend that the 
expression "nint(t*100.0)" actually has values between 17300 and 37300, 
so that the nint could be evaluated as int(t*100.0 + 0.5) instead of the 
far more complicated expression due to Fortran 95's semantics (which of 
course have to cater for negative numbers).

On our Sun, just being able to inline the nint got us a 30 % improvement 
in run time (over all) - so it's obvious what halving the number of 
instructions will buy us.

--
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
Maintainer, GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
GNU Fortran 95: http://gcc.gnu.org/fortran/ (under construction)